The present invention relates to solid state amplifiers, and more particularly to an amplifier circuit having a high input impedance and high input voltage range.
In measuring circuits, it is sometimes necessary to present a very high input impedance to the circuit being measured so as not to affect its performance. Also buffer amplifiers are required to provide isolation between a high impedance transducer, such as a condenser microphone or an electrostatic accelerometer, and its load. Such high impedance requirements are typically limited to insulated gate field effect transistors (IGFET) and junction effect transistors (JET) as sensing devices, replacing the previous vacuum tube cathode follower and transistor emitter follower circuits.
For ultra low leakage current applications, the IGFET is the most desirable choice. Since these devices are usually limited to 15-30 volt power supply voltages, maximum, a usual measurement limitation is to impose a few dozen volts working voltage range. Thus, for applications where a measurement range of hundreds or thousands of volts is needed, these devices are not suitable.